1. Field of the Invention
The present invention relates to a system of compression/decompression of animated images, especially according to standards H.261 and H.263. The present invention more particularly relates to the subset of such a system used for processing of streams of coded bits transmitted between two systems.
2. Discussion of the Related Art
FIG. 1 schematically shows such a subset 10. This subset includes a variable-length coding (VLC), or Huffman, block 12 providing variable-length codes to a framing block 13. This block 13 issues a series flow of bits meant to be decoded by a reception circuit.
The VLC block generally receives data resulting from a run-level coding (RLC) 14. Such a coding consists in transforming a chain of zeros followed with a non-zero value, the first one of which ("run", or chain) indicates the number of zeros preceding the non-zero value, and the second one of which ("level") provides the non-zero value.
A reception circuit, also included in subset 10, includes a frame processing (unframing) block 15 followed with a variable-length decoding (VLD) block 16 to perform the operations reciprocal to framing 13 and VLC 12.
VLD 16 is generally followed with a run-level decoding (RLD) 17, reciprocal to the RLC.
A VLC or Huffman coding consists of replacing each run-level couple with a code, the length of which (from 2 to 12 bits for standards H.261 and H.263) is all the shorter as the probability of occurrence of the couple is large. These variable-length codes are fetched in correspondence tables and concatenated to form a sequence of contiguous codes. The function of framing circuit 14 is to insert the concatenated codes into frames including various informations, especially error correction codes. These frames are defined by standards H.261 and H.263 or by standards H.221 and H.223 which can be used together with standards H.261 and H.263.
The operations of subset 10 are generally performed in a wired manner. Indeed, if a microprocessor of general use programmed to perform these operations was desired to be used, the processing power would be too much for the cost of the microprocessor to be reasonable. This remains true in the context of standard H.263 where the bit streams have a particularly low rate (adapted to the telephone lines of the switched network).
Yet, the use of a microprocessor to perform the operations of subset 10 would be especially advantageous since it would enable a particularly easy adaptation to variations of the standards by a simple program modification without having to entirely redesign the subset.